Foodborne salmonellosis is a major public health problem. Salmonella is the second leading cause of annual foodborne illness cases, with an estimated 1,340,000 cases compared with nearly 2,000,000 for Campylobacter. A survey conducted in the United States estimated that in 1999 non-typhoidal salmonellosis of foodborne origin caused approximately 15,600 hospitalizations and 550 deaths (Mead, P. S., L. Slutsker, V. Dietz, et al. Food-related illness and death in the United States. Emerg. Infect. Dis. 5:607-625. 1999). Although the genus Salmonella has more than 2300 serovars, only a relatively restricted number belonging mainly to the Enterica subspecies of the Salmonella enterica species are responsible for the great majority of human infections. The most common sources of Salmonella infections in humans are contaminated foods, including eggs, poultry, produce, meat, and meat products. Eggs and poultry meat are recognized as the major vehicles of human infections because of epizootics in fowl.
Early detection of foodborne Salmonella is vital for food safety assurance. However, conventional methods for detecting foodborne Salmonella are laborious and time consuming. These methods typically involve identifying presumptively positive samples by sequentially processing samples in a pre-enrichment phase, a selective-enrichment phase, and then an analysis phase. The analysis phase may involve culturing the enriched sample on selective differential agar, analyzing with polymerase chain reaction (PCR), and/or analyzing via immunoassay. After identifying the presumptively positive samples, confirmation of the presumptively positive samples typically requires biochemical characterization of isolates obtained from selective, differential agar media. See, e.g., U.S. Food and Drug Administration (2011) Bacteriological Analytical Manual, Chapter 5. It is estimated that millions of such Salmonella analyses are run routinely in the United States each year.
Early detection of Salmonella is needed if foodborne illnesses caused by Salmonella are to be reduced. While determinative microbiology requires confirmation of presumptively positive samples, this is not the case in many applications such as environmental monitoring and food safety-HACCP testing. In such applications, only a reasonable presumption that a sample is contaminated is required to take such corrective actions as modifying a sanitation procedure or quarantining a product lot pending subsequent confirmation. Rapid screening tests which have comparable diagnostic performance to culture methods or other rapid molecular methods can suffice.
Several Salmonella-selective enrichment media and systems are known in the art. See, e.g., U.S. Pat. No. 4,279,995 to Woods et al.; U.S. Pat. No. 5,208,150 to Tate et al.; U.S. Food and Drug Administration (2011) Bacteriological Analytical Manual, Chapter 5; U.S. Pat. No. 7,704,706 to Druggan; U.S. Pat. No. 7,150,977 to Restaino; U.S. Pat. No. 6,368,817 to Perry et al.; U.S. Pat. No. 5,434,056 to Monget et al.; and U.S. Pat. No. 5,194,374 to Rambach. The media and systems described in these references do not provide sufficient selectivity of Salmonella spp.
Another major cause of foodborne illness is Shiga toxin-producing E. coli (STEC). STEC has been linked with the severe complication hemolytic-uremic syndrome (HUS) and other deleterious outcomes. Shiga toxin-producing E. coli is known by a number of other names, including enterohemorrhagic E. coli (EHEC), Shiga-like toxin-producing E. coli (SLTEC), hemolytic uremic syndrome-associated enterohemorrhagic E. coli (HUSEC), and verocytotoxin- or verotoxin-producing E. coli (VTEC).
All the Shiga toxin-producing E. coli strains produce Shiga toxin (also known as, Shiga-like toxin or verotoxin), a major cause of foodborne illness. EHEC strains are distinguished from commensal E. coli. Shiga toxin-producing E. coli strains can be distinguished from non-Shiga toxin-producing E. coli strains by detecting the Shiga toxin genes (Stx1 and/or Stx2) in E. coli or the gene products thereof. See, e.g., Chassagne L, Pradel N, Robin F, Livrelli V, Bonnet R, Delmas J. Detection of stx1, stx2, and eae genes of enterohemorrhagic Escherichia coli using SYBR Green in a real-time polymerase chain reaction. Diagn Microbiol Infect Dis. 2009 May; 64(1):98-101.
The best known of Shiga toxin-producing E. coli strains is O157:H7, but non-O157 strains cause an estimated 36,000 illnesses, 1,000 hospitalizations and 30 deaths in the United States yearly. Food safety specialists recognize the “Big Six” Shiga toxin-producing E. coli strains: O26, O45, O103, O111, O121, and O145. A 2011 outbreak in Germany was caused by another Shiga toxin-producing E. coli strain, O104:H4. This strain has both enteroaggregative and enterohemorrhagic properties. Both the O145 and O104 strains can cause hemolytic-uremic syndrome. The former strain was shown to account for 2% to 51% of known HUS cases. An estimated 56% of such cases are caused by O145 and 14% by other Shiga toxin-producing E. coli strains.
Shiga toxin-producing E. coli strains that induce bloody diarrhea lead to HUS in 10% of cases. The clinical manifestations of postdiarrheal HUS include acute renal failure, microangiopathic hemolytic anemia, and thrombocytopenia. The Shiga toxin can directly damage renal and endothelial cells. Thrombocytopenia occurs as platelets are consumed by clotting. Hemolytic anemia results from intravascular fibrin deposition, increased fragility of red blood cells, and fragmentation.
An important trend in the meat industry in the United States is the ability to cultivate E. coli and Salmonella in the same selective enrichment media. Laboratories would like to streamline their processes by reducing the steps to regulatory compliance to one operation for screening for both pathogenic E. coli and Salmonella. 
There exists a need for a simple, rapid screening test that identifies presumptively positive Salmonella and/or Shiga toxin-producing E. coli samples at an early stage of the sample analysis, preferably in a selective enrichment stage of the analysis. Simultaneous enrichment and detection of Salmonella and/or Shiga toxin-producing E. coli using a single testing method would reduce not only time but also the cost of labor and media. Streamlining procedures and reducing labor and test costs should permit more frequent monitoring for Shiga toxin-producing E. coli, thereby reducing contamination hazard.